Control system for a fuel burner

ABSTRACT

An electrically energizable control system for a fuel burner where a fuel valve is opened on thermostatic demand and closed a predetermined time thereafter if ignition fails. In the control system, a Darlington amplifier is provided for flame detection, and a spark igniter supply circuit has a transformer coupled first and second gate controlled switches. A second transformer is coupled to the second gate controlled switch for providing a source of sparks for igniting fuel. A spark igniter supply circuit, a flame detector circuit as well as several other safety features are also disclosed.

'Wyland v [451 Nov. 18, 1975 CONTROL SYSTEM FOR A FUEL BURNER Primary ExaminerEdward G. Favors Attorney, Agent, or FirmJoseph E. Papin [75] Inventor: Alvin D. Wyland, Morrison, Ill.

[73] Assignee: General Electric Company, Fort Wayne, Ind. [57] ABSTRACT Filedi J y 1974 An electrically energizable control system for a fuel burner where a fuel valve is o ened on thermostatic 21 A 1.N.:486 04 P l l PP O ,0 demand and closed a predetermined time thereafter if ignition fails. In the control system, a Darlington am- [52] US. Cl. 431/79; 431/69 plifier is provided for flame detection, and a Spark ig- [51] Int. Cl. F23H 5/08 niter supply circuit has a transformer coupled first and Field of Search 79, 0, 8, 71 second gate controlled switches. A second transformer is coupled to the second gate controlled switch for [56] References Cited providing a source of sparks for igniting fuel.

' UNITED STATES PATENTS A spark igniter supply circuit, a flame detector circuit 3,627,458 12/1971 Wade 431/78 as well as several other safety features are also 3,767,354 10/1973 Wright disclosed.

3,770,365 11/1973 Lenski 7' 18 Claims, 4 Drawing Figures I w -3I Q 4 as l 49 l 101 Q? I [\L l i 53 I 149 r, I v

Patent Nov. ,18, 1975 Sheet 1 of4 3,920,376

FlG.1a

Patent Nov. 18, 1975 Sheet 3 of 4 US. Patent Nov. 18, 1975 Sheet40f4 3,920,376

1 1 CONTROL SYSTEM FOR A FUEL BURNER BACKGROUND OF THE INVENTION The present invention relates generally to a control system for a fuel burner and in particular to a low voltage control system of the direct ignition type.

As is well known in the prior art, fuel burners, such as gas furnaces for instance, typically provide a thermostat to open a fuel valve when that thermostat demands heat and some means for igniting the fuel emitted from the burner. Ignition may be achieved by a small pilot flame which burns continuously or by using a spark or are igniter which provides a spark in the vicinity of the burner soon after the fuel valve is opened.

An improvement in burner control units of the above-mentioned type is illustrated in my US. Pat. No. 3,734,676 which discloses a control system for a fuel burner. In this patented control system, an igniter and a controlled switching device are effective, when gated, to connect the igniter to a source of electrical energy for providing pulses of electrical energy to the igniter. My prior patented control system has a gate circuit including a capacitance for providing gate signals to the controlled switching device. These gate signals are provided so long as the capacitance has a charge below a predetermined level and the capacitance is initially provided with a charge of at least the predetermined level. This initial charge is reduced below the predetermined level after the expiration of a predetermined period of time. In this manner, the controlled switching device is gated to operatively connect the igniter to a source of electrical energy only after a predetermined period of energization of the control system thereby to effect pre-ignition purging of the burner area to insure there is not an overabundance of fuel when ignition occurs.

Control systems according to my prior patent function well but operate on a 120 volt alternating current supply or employ a step-up transformer from the commonly encountered low voltage thermostat systems which typically operate around 24 volts A.C.

SUMMARY OF THE INVENTION Among the several objects of the present invention may be notedthe provision of an improved solid state ignition and flame monitoring system for a fuel burner; the provision of a low voltage operating electrically energizable control system for a fuel burner; the provision of a fuel burner system having a plurality of safety features incorporated therein; the provision of an improved spark igniter circuit which may be utilized with a control system for a fuel burner; the provision of an improved flame detector circuit which may also be utilized with a control system for a fuel burner; and the provision of a fuel burner control system, a flame detector circuit, and a spark igniter circuit which are respectively simplistic in design, economical to manufacture, and easily assembled and maintained. Other objects and features will be in part apparent and in part pointed out hereinafter.

In general and in one form of the invention, a control system for a fuel burner has a fuel valve which is opened on thermostatic demand and closed a predetermined time thereafter if ignition fails. The.control system includes a flame detector circuit having a Darlington amplifier and means in circuit relation with the Darlington amplifier input for enabling it when impedance between the enabling means and a reference potential is below a predetermined impedance. The enabling means is operable generally to disable the Darlington amplifier when impedance between the enabling means and the reference potential is above the predetermined impedance. A spark ignitor supply circuit is also provided including means operable generally for providing a spark to effect ignition of the fuel supplied to the fuel burner and means for controlling the spark providing means to effect its operation in the event the enabling means disables the Darlington amplifier.

Also in general, an electrically energizable control system in one form of the invention is provided for a fuel burner. The system has means for actuating a fuel valve to enable flow of combustible fuel from the fuel burner, and means is provided for igniting fuel emitted from the fuel burner. A first controlled switching device is provided in the system, and means is provided for connecting the actuating means and first controlled switching device to a source of electrical energy to effect energization of the actuating means when the first controlled switching device becomes conductive. A gate controlled switch is coupled to the first controlled switching device for rendering it nonconducting when the gate controlled switch has been nonconducting for a predetermined length of time, and a third controlled switching device is responsive to the combustion of fuel emitted from the burner for gating the gate controlled switch to a conducting state.

Further in general and in one form of the invention, an electrically energizable control system is provided fora fuel burner having a fuel valve opened on thermostatic demand and closed a predetermined time thereafter if ignition fails. An improved spark igniter supply circuit is provided in the system and includes first and second gate controlled switches, and first and second transformer means. One winding of the first transformer means is connected in series with the first gate controlled switch, and another winding of the first transformer means is coupled to the gate of the second gate controlled switch. Also, one winding of the second transformer means is connected in series with the second gate controlled switch, and another winding of the second transformer means provides a spark source for igniting fuel emitted from the burner.

Still further and in general, an electrically energizable control system for a fuel burner is provided having a fuel valve opened on thermostatic demand and closed a predetermined time thereafter if ignition fails. An improved flame detector circuit is provided in the system having a Darlington amplifier. Means is in circuit relation with the Darlington amplifier input for enabling it when impedance between the enabling means and a reference is below a predetermined impedance, and the enabling means is also operable generally to disable the Darlington amplifier when the impedance between the enabling means and the reference potential is above the predetermined impedance. A spark ignitor circuit has first and second gate controlled switches and first and second transformer means. One winding of the first transformer means is connected in series with the first gate controlled switch, and another winding of the first transformer means is coupled to the gate of the second gate controlled switch. One winding of the second transformer means is connected in series with the secnd gate controlled switch, and another winding of the second transformer means provides a spark source for igniting the fuel. The Darlington amplifier output is coupled to the gate of the first gate controlled switch to maintain it in a conducting state so long as the thermostat demands and a flame is present.

In general, an electrically energizable control system in one form of the invention is provided for a fuel burner. The system has means for actuating a fuel valve to enable flow of a combustible fuel to the burner and also means for igniting the fuel emitted from the burner. A first controlled switching device is provided in the system, and means aare provided for connecting the first controlled switching device to a source of electrical energy to effect energization of the actuating means when the first controlled switching device becomes conductive. A second controlled switching device is coupled to the first controlled switching device for rendering it nonconducting when the second controlled switching device has been nonconducting for a predetermined length of time. A third controlled switching device is responsive to combustion of the fuel emitted from the burner for maintaining the second controlled switching device in its conducting state. The first controlled switching device includes a relay having normally open contacts in series with the actuating means and the source of electrical energy and also an actuating coil in series with the second controlled switching device.

BRIEF DESCRIPTION OF THE DRAWING FIGS. la and 1b when joined together form a schematic diagram of a low voltage control system according to the present invention; and

FIGS. 2a and 2b when jointed together form a schematic diagram of a low voltage control according to the present invention incorporating additional lock-out features. I

Corresponding reference characters indicate corresponding parts throughout the several views of the drawing.

The following examples illustrate the invention and are not to be construed as limiting in any manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing in greater detail, there is indicated generally at in FIGS. 1a and lb a control system which receives low voltage, such as 24 volts alternating current for instance, between a pair of terminals 11 and 13. Upon a demand for heat, a thermostatic switch 15, of a type well known in the art such as a bimetal thermostat or the like, closes thereby to supply this low voltage to the remainder of the circuit. A rectified charging current flows into a capacitor voltage divider circuit 17, 19, and when sufficient charge accumulates on capacitor 19, a relay actuating coil 21 is energized sufficiently to close its normally open contacts 23. If a gate controlled switch 25, which may, for example, be a silicon controlled rectifier or the like, is in its conducting state, a relay actuating coil 27 conducts sufficient current by way of contacts 23, a diode 29, a resistor 31, silicon controlled rectifier and a transformer primary winding 33 to close its normally open contacts 35, 37. When closed, contacts 37 supply electrical energy (which may or may not be from the same source as the energy source coupled to terminals 11,

13) to a valve actuating device 39 which opens thereby to allow flow of fuel to a burner 41. Gate current for enabling controlled switching device 25 is provided by way of contacts 23 or 35, a resistor 43, a resistor 45, a diode 47, a capacitor 49, a switching device 51 (which may be a diac or, as disclosed in my aforementioned patent, a neon tube or other device having a minimum voltage conduction trait), and a diode 53. Parallel paths also exist by way of a diode 55, a resistor 57, and a diode 59 as well as diode 29, relay coil 27, and resistor 31. In one preferred implementation with the assumed 24 volt alternating current energization of terminals 11, 13, the voltage across gate controlled switch 25 must attain a selected voltage, generally about 8-l6 volts, before diac 151 conducts to trigger switch 25. Under these circumstances, there is also a similar voltage charge on a capacitor 61, and when switch 25 conducts, this capacitor is discharged through primary winding 33 of a transformer T1 which, in turn, induces a voltage in a secondary winding 63 of the transformer to trigger another controlled switching device, such as a silicon controlled rectifier 65. Conduction by switch 65 discharges the previously accumulated charge on a capacitor 67 through a primary winding 69 of another transformer T2 which functions as a step-up transformer having a secondary winding 71 connected between ground and a spark probe 73 to provide an igniting spark in the burner 41.

In this manner, each time the silicon controlled rectifier 25 conducts, it draws a certain amount of gate current thus incrementing the charge on capacitor 49 and decreasing the portion of the line voltage available to break down diac 51. When the charge on capacitor 49 increments sufficiently to prevent the breakdown of diac 51, silicon controlled rectifier 25 no longer receives gating signals and ceases to provide ignition pulses. This ignition trial time may be of the order of 10 seconds. After the ignition trial, if no flame has been established, the lack of conduction of silicon controlled rectifier 25 interrupts the energizing current for relay coil 27, and as soon as the charge on a capacitor 75 discharges through the relay coil that relay will open its contacts 35, 37 thus shutting off the fuel flow to burner 41.

As well known in the art, a flame has an impedance substantially lower than the impedance of a corresponding air gap, and the flame impedance is a function of the direction of current flow, as discussed hereinafter. In FIG. la, a detector probe 75 and a grounded conductor 77 are placed near burner 41 so that when a flame is established at the burner, the impedance between detector probe 75 and grounded conductor 77 is, for example, about 10 megohms. Ifa flame has been established in burner 41, the flame current flow causes a corresponding base current in a transistor Q1, which in turn induces a substantial base current in a transistor Q2 so that the gate of silicon controlled rectifier 25 may now be held constantly on due to the supply of substantially constant direct current from a filtered half-wave supply formed by a diode 79, a capacitor 81,

and a resistor 83. Transistors Q1 and Q2 are coupled together as a Darlington amplifier and will function much like a switch. Therefore, when no flame is present, transistor O2 is substantially non-conducting, and when the flame impedance appears causing transistor O1 to conduct, transistor Q2 becomes substantially saturated thus holding silicon controlled rectifier 25 in its conducting state.

Since silicon controlled rectifier 25 is receiving a gating signal when the alternating current passes through zero voltage, capacitor 61 will not charge, and there will be no pulse applied to primary winding 33 of transformer Tl sufficient to trigger silicon controlled rectifier 65. Thus the ignitor circuit is disabled. Capacitor 49 will discharge by way of a diode 85, a resistor 87, and a diode 89 under this flame present condition.

If for some reason the flame should become extinguished, transistors Q1, Q2 immediately cease conducting, and the anode to cathode voltage across silicon controlled rectifier 25 will again build up to the exemplary 8-16 volt level. Further, transformer primary winding 33 will again receive pulses when switch 25 conducts to again trigger the igniting circuit. These above discussed ignition trials will continue until the flame is re-established or the charge on capacitor 49 builds up sufficiently to lock-out.

The ignition spark is generated by discharging capacitor 67 through primary winding 69 of transformer T2. While transformer T1 could, for example, be a one-toone isolation transformer, transformer T2 has a high turns ratio to generate a high voltage pulse in secondary winding 71 sufficient to jump the desired spark gap. Capacitor 67 is a portion of a voltage doubler which also includes a capacitor 91, a pair of diodes 93, 95, and a resistor 97. When terminal 13 is positive relative to terminal 11, current flows generally downwardly through resistor 97 and diode 93 to charge capacitor 91 up to substantially peak line voltage. When terminal 13 is negative relative to terminal 11, current flows through the series combination, as at 91, 67 generally upwardly through a diode 95 and resistor 97. With respect to the voltage doubler circuits, this effectively places a voltage on capacitor 67, with the lower plate thereof positive, which is close to but slightly less than twice the peak value of the line voltage. When silicon controlled rectifier 65 is triggered by a pulse transmitted through transformer T1, a charge on capacitor 67 discharges through primary winding 69 of step-up transformer T2 providing on secondary winding 71 a desired spark voltage.

In view of the foregoing discussion, the function of several other circuit elements in the schematic diagram of FIGS. 1a and lb should be clear. Briefly, resistors 31, 45, 83, 97 function as current limiting resistors while resistors 57, 87, 99 provide capacitor discharge paths during the periods when thermostat is not supplying power to the circuit. Capacitors 19, 75, 81, 101 function as filter capacitors, and it may be noted that capacitor 75 maintains relay coil 27 energized during short intervals when silicon controlled rectifier is not conducting.

If silicon controlled rectifier 25 is shorted or gated into its conducting state, for example, by the detector probe 75 being grounded or transistor Q2 shorted out when the voltage is initially applied to the circuit by the closing of thermostat l5, coil 21 will not be energized, and contact 23 will not close thus insuring relay coil 27 is not energized and that the fuel valve will remain in its closed position.

The safety feature discussed in the previous paragraph along with the advantages of the circuit of FIGS. 1a and 1b which include less costly components due to the lower operating voltage as well as other advantages earlier discussed may also be found in another system in one form of the invention, as indicated generally at 102 in FIGS. 2a and 2b. System 102 may function generally in the manner as system I discussed hereinabove with respect to FIGS. 1a and lb, but it may be noted that system 102 has additional safety features as well as additional advantageous features of its own.

Referring now with particularity to FIGS. 2a and 2b, the circuit of system 102 provides a pre-start safety check of a principal control element, such as a silicon controlled rectifier 103, as well as the lock-out functions with relative certainty that an opened or short circuited component will not create an unsafe condition.

When the low voltage alternating current is applied to the circuit of system 102 due to a thermostat 105 calling for heat, a capacitor 107 charges-until its voltage reaches the breakdown voltage for a diac 109, which is one preferred implementation was about 8 volts. When diac 109 conducts, a light emitting diode 111 is energized, and the light thereof is coupled to a light activated silicon controlled rectifier or switch 113 to enable it. In this manner, rectified line voltage is applied to a relay actuating coil 115 which functions in a latching mode to close its contacts 117 as well as contacts 119 thereby to energize a fuel valve 121 to supply fuel to a burner l23. Light emitting diode 111 and light activated silicon controlled rectifier 113 may be of a type available on a single small integrated circuit chip (not shown). If for some reason capacitor 107 cannot charge up to the breakdown voltage for diac 109, no coil current will flow in relay actuating coil 115 thereby to prevent fuel flow to burner 123. Capacitor 107 will be unable to charge if silicon controlled rectifier 103 is shorted, a flame detector probe 125 is grounded, one of a pair of transistor Q4, O5 is shorted, or a capacitor 127 is shorted. In practice, the ratio of the resistances of a resistor 129 to a resistor 131 are about 60 to 1, and if a diode 132 is for any reason shorted, the current through relay actuating coil 115 is limited to less than the hold-in value for that relay thus prohibiting fuel flow.

A Darlington pair 06, which, like the other Darlington configured transistor pairs in this application, may be a physical separated pair of transistors or an integral amplifier. Darlington pair Q6 is in the circuit to provide an additional lock-out function, and capacitor 133 incrementally increases its charge just as the previously discussed capacitor 49 of FIG. 1a. When this charge is sufficient that a diac 135 no longer conducts to provide ignition pulses to the ignition probe, a lock-out occurs disabling relay actuating coil 115 and shutting off the fuel flow. If, for some reason, capacitor 133 has substantial leakage or otherwise fails to perform this lockout function, transistor Q6 will do so at a later time. Transistor Q6 is nonconducting until the voltage on a capacitor 137 reaches the zener-diode voltage of a zener diode 139, at which time Darlington pair Q6 switches to its conducting state and essentially grounds the gate of silicon controlled rectifier 103 thus shutting down the system. The lock-out time for transistor Q6 is greater than that associated with the timing circuitry including capacitor 133 and is essentially a function of the value of capacitor 137 and of a resistor 141.

In other respects, the circuit of system 102 functions generally in the same manner as system 1 previously discussed with respect to FIGS. 1a and lb. A pair of diodes, such asindicated at 143, 145, (147 and 149 in H08. la and lb) are present to conduct when the fields in their respective transformers collapse. A capacitor 151 provides an alternating current filtering function for the flame detector probe.

While no specific mention has been made of the manner in which the circuits are reset once a lock-out condition occurs, it should be clear that such lock-out is caused by an accumulated charge on one or more capacitors and that reset may be achieved by, for example, manually opening thermostat 105 and either relying on the inherent leakage in those capacitors or providing bleeder resistances to bleed off the charge prior to re-energizing the circuit.

It may be noted that many of the features of my aforementioned patent, as well as other prior art devices, may be incorporated into the control system of the present invention. For instance, a preignition time delay for purging the fuel burner area of excess fuel has not been discussed herein, but such is clearly within the scope of the present invention.

From the foregoing, it is now apparent that novel control systems 10, 102 as well as a novel spark igniter circuit and a novel flame detector circuit for use therein are presented meeting the objects and advantageous features set out hereinbefore, as well as others. Further, it is also contemplated that changes as to the arrangements, details and connections of the component parts of the novel control systems, flame detector circuit, and spark igniter circuit, which are presented to illustrate the invention, may be made by those skilled in the art without departing from the spirit or scope of the invention as set out in the claims which follow.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrically energizable control system for a fuel burner comprising:

means for actuating a fuel valve to enable flow of a combustible fuel to the burner;

means for igniting the fuel emitted from the burner;

a first controlled switching device;

means for connecting the actuating means and the first controlled switching device to a source of electrical energy to effect energization of the actuating means when the first controlled switching device becomes conductive;

a second controlled switching device coupled to the first controlled switching device for rendering the first controlled switching device nonconducting when the second controlled switching device has been nonconducting for a predetermined length of time;

a third controlled switching device responsive to combustion of the fuel emitted from the burner for maintaining the second controlled switching device in its conducting state; and

the first controlled switching device including a relay having normally open contacts in series with the actuating means and the source of electrical energy, and an actuating coil in series with the second controlled switching device.

2. A control system as set forth in claim 1, further comprising means responsive to an initial conducting state of the second switch and nonconducting state of the actuating coil for preventing the actuation of the relay.

3. In an electrically energizable control system for a fuel burner having a fuel valve opened on thermostatic demand and closed a predetermined time thereafter if fuel ignition fails; a spark igniter supply circuit comprising first and second gate controlled switches, first and second transformers, one winding of the first transformer being connected in series with the first gate controlled switch and another winding of the first transformer being coupled to the gate of the second gate controlled switch, and one winding of the second transformer being connected in series with the second gate controlled switch and another winding of the second transformer providing a spark source for igniting the fuel.

4. The circuit as set forth in claim 3, further comprising means for providing a series of enabling signals to the gate of the first gate controlled switch, means for stopping the enabling signals a predetermined time after the enabling signals begin.

5. The circuit as set forth in claim 4, further comprising a timing circuit coupled to the gate of the first gate controlled switch to render it nonconducting at a time subsequent to the predetermined time after the enabling signals begin.

6. The circuit as set forth in claim 3, further comprising a relay having normally open contacts closable to open the fuel valve and having an actuating coil in series with the first gate controlled switch.

7. The circuit as set forth in claim 6, further comprising means responsive to an initial conducting state of the gate controlled switch and nonconducting state of the actuating coil for preventing actuation of the relay.

8. In an electrically energizable control system for a fuel burner where a fuel valve is opened on thermostatic demand and closed a predetermined time thereafter if fuel ignition fails; a flame detector circuit comprising a Darlington amplifier, means in circuit relation with the Darlington amplifier input for enabling it when impedance between the enabling means and a reference potential is below a predetermined impedance, the enabling means also being operable generally to disable the Darlington amplifier when impedance between the enabling means and the reference potential is above the predetermined impedance, a spark igniter supply circuit including first and second gate controlled switches, and first and second transformer means, one winding of the first transformer means being connected in series with the first gate controlled switch and another winding of the first transformer means being coupled to the gate of the second gate controlled switch, and one winding of the second transformer means being connected in series with the second gate controlled switch and another winding of the second transformer means providing a spark source for igniting the fuel, the output of the Darlington amplifier being coupled to the gate of the first gate controlled switch to maintain it in a conducting state so long as the thermostat demands and a flame is present.

9. The circuit as set forth in claim 8, further comprising means for providing a series of enabling signals to the gate of the first gate controlled switch, means for stopping the enabling signals a predetermined time after the enabling signals begin, and a timing circuit coupled to the gate of the first gate controlled switch to render that switch nonconducting at a time subsequent to the predetermined time after the enabling signals begin.

10. The circuit as set forth in claim 8, further comprising a relay having normally open contacts closable to open the fuel valve and having an actuating coil in series with the first gate controlled switch, and means responsive to an initial conducting state of the gate controlled switch and nonconducting state of the actuating coil for preventing actuation of the relay.

11. An electrically energizable control system for a fuel burner comprising: means for actuating a fuel valve to enable flow of a l combustible fuel to the burner;

means for igniting the fuel emitted from the burner;

a first controlled switching device;

means for connecting the actuating means and the first controlled switching device to a source of electrical energy to effect energization of the actuating means when the controlled switching device becomes conductive;

gate controlled switch coupled to the first controlled switching device for rendering it nonconducting when the gate controlled switch has been nonconducting for a predetermined length of time; and

a Darlington amplifier responsive to combustion of the fuel emitted from the burner for enabling the gate of the gate controlled switch to maintain it in a conducting state.

12. An electrically energizable control system for a fuel burner comprising:

means for actuating a fuel valve to enable flow of a combustible fuel to the burner;

means for igniting the'fuel emitter from the burner;

a first controlled switching device;

means for connecting the actuating means and the first controlled switching device to a source of electrical energy to effect energization of the actuating means when the controlled switching device becomes conductive;

gate controlled switch coupled to the first controlled switching device for rendering it nonconducting when the gate controlled switch has been nonconducting for a predetermined length of time; and

a" third controlled switching device responsive to combustion of the fuel emitted from the burner for gating the gate controlled switch to a conducting state.

13. A control system as set forth in claim 12, further comprising means coupled to the gate of the gate controlled switch for also supplying enabling signals to the gate.

14. A control system as set forth in claim 13, further comprising means responsive to the enabling of the gate controlled switch by the supplying means for actuating the igniting means to ignite the fuel.

15. A control circuit as set forth in claim 13, further comprising a first timing circuit for controlling the supplying means to stop the enabling signals thereof a predetermined time after the enabling signals begin.

16. A control system as set forth in claim 15, further comprising a second timing circuit coupled to the gate of the gate controlledswitch to render it nonconductive at a time subsequent to the predetermined time after the enabling signals begin.

17. A control system as set forth in claim 12, wherein the actuating means comprises a first transformer, another gate controlled switch, and a second transformer, one winding of the first transformer being connected in series with the first named gate controlled switch and another winding of the first transformer being coupled to the gate of the other gate controlled switch, and one winding of the second transformer being connected in series with the other gate controlled switch and another winding of the second transformer being coupled to the igniting means. w

18. In an electrically energizable control system for a fuel burner where a fuel valve is opened on thermostatic demand and closed a predetermined timethereafter if fuel ignition fails; the combination therewith comprising a flame detector circuit including a Darlington amplifier, and means in circuit relation with the Darlington amplifier input for enabling it when impedance between the enabling means and a reference potential is below a predetermined impedance, the enaabling means also being operable generally to disable the Darlington amplifier when impedance between the enabling means and the reference potential is above the predetermined impedance, and a spark ignitor supply circuit including means operable generally for providing a spark to effect ignition of the fuel supplied by the fuel burner, and a gate controlled switch for controlling the spark providing means to effect its operation in the event the enabling means disables the Darlington amplifier. 

1. An electrically energizable control system for a fuel burner comprising: means for actuating a fuel valve to enAble flow of a combustible fuel to the burner; means for igniting the fuel emitted from the burner; a first controlled switching device; means for connecting the actuating means and the first controlled switching device to a source of electrical energy to effect energization of the actuating means when the first controlled switching device becomes conductive; a second controlled switching device coupled to the first controlled switching device for rendering the first controlled switching device nonconducting when the second controlled switching device has been nonconducting for a predetermined length of time; a third controlled switching device responsive to combustion of the fuel emitted from the burner for maintaining the second controlled switching device in its conducting state; and the first controlled switching device including a relay having normally open contacts in series with the actuating means and the source of electrical energy, and an actuating coil in series with the second controlled switching device.
 2. A control system as set forth in claim 1, further comprising means responsive to an initial conducting state of the second switch and nonconducting state of the actuating coil for preventing the actuation of the relay.
 3. In an electrically energizable control system for a fuel burner having a fuel valve opened on thermostatic demand and closed a predetermined time thereafter if fuel ignition fails; a spark igniter supply circuit comprising first and second gate controlled switches, first and second transformers, one winding of the first transformer being connected in series with the first gate controlled switch and another winding of the first transformer being coupled to the gate of the second gate controlled switch, and one winding of the second transformer being connected in series with the second gate controlled switch and another winding of the second transformer providing a spark source for igniting the fuel.
 4. The circuit as set forth in claim 3, further comprising means for providing a series of enabling signals to the gate of the first gate controlled switch, means for stopping the enabling signals a predetermined time after the enabling signals begin.
 5. The circuit as set forth in claim 4, further comprising a timing circuit coupled to the gate of the first gate controlled switch to render it nonconducting at a time subsequent to the predetermined time after the enabling signals begin.
 6. The circuit as set forth in claim 3, further comprising a relay having normally open contacts closable to open the fuel valve and having an actuating coil in series with the first gate controlled switch.
 7. The circuit as set forth in claim 6, further comprising means responsive to an initial conducting state of the gate controlled switch and nonconducting state of the actuating coil for preventing actuation of the relay.
 8. In an electrically energizable control system for a fuel burner where a fuel valve is opened on thermostatic demand and closed a predetermined time thereafter if fuel ignition fails; a flame detector circuit comprising a Darlington amplifier, means in circuit relation with the Darlington amplifier input for enabling it when impedance between the enabling means and a reference potential is below a predetermined impedance, the enabling means also being operable generally to disable the Darlington amplifier when impedance between the enabling means and the reference potential is above the predetermined impedance, a spark igniter supply circuit including first and second gate controlled switches, and first and second transformer means, one winding of the first transformer means being connected in series with the first gate controlled switch and another winding of the first transformer means being coupled to the gate of the second gate controlled switch, and one winding of the second transformer means being connected in series with the second gate controlled switch and another winding of thE second transformer means providing a spark source for igniting the fuel, the output of the Darlington amplifier being coupled to the gate of the first gate controlled switch to maintain it in a conducting state so long as the thermostat demands and a flame is present.
 9. The circuit as set forth in claim 8, further comprising means for providing a series of enabling signals to the gate of the first gate controlled switch, means for stopping the enabling signals a predetermined time after the enabling signals begin, and a timing circuit coupled to the gate of the first gate controlled switch to render that switch nonconducting at a time subsequent to the predetermined time after the enabling signals begin.
 10. The circuit as set forth in claim 8, further comprising a relay having normally open contacts closable to open the fuel valve and having an actuating coil in series with the first gate controlled switch, and means responsive to an initial conducting state of the gate controlled switch and nonconducting state of the actuating coil for preventing actuation of the relay.
 11. An electrically energizable control system for a fuel burner comprising: means for actuating a fuel valve to enable flow of a combustible fuel to the burner; means for igniting the fuel emitted from the burner; a first controlled switching device; means for connecting the actuating means and the first controlled switching device to a source of electrical energy to effect energization of the actuating means when the controlled switching device becomes conductive; a gate controlled switch coupled to the first controlled switching device for rendering it nonconducting when the gate controlled switch has been nonconducting for a predetermined length of time; and a Darlington amplifier responsive to combustion of the fuel emitted from the burner for enabling the gate of the gate controlled switch to maintain it in a conducting state.
 12. An electrically energizable control system for a fuel burner comprising: means for actuating a fuel valve to enable flow of a combustible fuel to the burner; means for igniting the fuel emitter from the burner; a first controlled switching device; means for connecting the actuating means and the first controlled switching device to a source of electrical energy to effect energization of the actuating means when the controlled switching device becomes conductive; a gate controlled switch coupled to the first controlled switching device for rendering it nonconducting when the gate controlled switch has been nonconducting for a predetermined length of time; and a third controlled switching device responsive to combustion of the fuel emitted from the burner for gating the gate controlled switch to a conducting state.
 13. A control system as set forth in claim 12, further comprising means coupled to the gate of the gate controlled switch for also supplying enabling signals to the gate.
 14. A control system as set forth in claim 13, further comprising means responsive to the enabling of the gate controlled switch by the supplying means for actuating the igniting means to ignite the fuel.
 15. A control circuit as set forth in claim 13, further comprising a first timing circuit for controlling the supplying means to stop the enabling signals thereof a predetermined time after the enabling signals begin.
 16. A control system as set forth in claim 15, further comprising a second timing circuit coupled to the gate of the gate controlled switch to render it nonconductive at a time subsequent to the predetermined time after the enabling signals begin.
 17. A control system as set forth in claim 12, wherein the actuating means comprises a first transformer, another gate controlled switch, and a second transformer, one winding of the first transformer being connected in series with the first named gate controlled switch and another winding of the first transformer being coupled to the gate of the oTher gate controlled switch, and one winding of the second transformer being connected in series with the other gate controlled switch and another winding of the second transformer being coupled to the igniting means.
 18. In an electrically energizable control system for a fuel burner where a fuel valve is opened on thermostatic demand and closed a predetermined time thereafter if fuel ignition fails; the combination therewith comprising a flame detector circuit including a Darlington amplifier, and means in circuit relation with the Darlington amplifier input for enabling it when impedance between the enabling means and a reference potential is below a predetermined impedance, the enaabling means also being operable generally to disable the Darlington amplifier when impedance between the enabling means and the reference potential is above the predetermined impedance, and a spark ignitor supply circuit including means operable generally for providing a spark to effect ignition of the fuel supplied by the fuel burner, and a gate controlled switch for controlling the spark providing means to effect its operation in the event the enabling means disables the Darlington amplifier. 